This invention relates to marine seismic exploration and particularly to a method of obtaining seismic traces from common reflection points in submerged geophysical formations which may be stacked to improve the signal to noise ratio.
Marine seismic reflection surveying is a well-known method of geological investigation which is widely employed to locate undersea geological formations which may contain natural gas or petroleum. Marine seismic reflection surveying is typically accomplished by towing a seismic wave source comprising one or more seismic wave generators such as airguns and a multiplicity of hydrophones or other suitable transducers in a trailing linear array over an area to be surveyed. As the source is towed over the survey area it is periodically activated to produce a seismic wave in the water which travels outward and downward through the seafloor and underlying strata. Portions of the wave energy are reflected back into the water by the seafloor and at each of the underlying strata interfaces. The returning reflected waves pass by the hydrophones or other transducers in the trailing sensor array and cause a disturbance in their outputted reflection signals from which the time of passage of the reflected wave can be determined. The propagation time of a seismic wave to and from a reflection point is directly related to the depth of that point. The sensor outputted reflection signals are subsequently processed to generate a topological representation of the subsurface formations for analysis. The seismic source and trailing sensor array are towed continually through the survey area while gathering the reflection data. This allows marine reflection surveys to be made of large areas rather quickly.
Seismic reflection data is most typically gathered by common depth point shooting. This is accomplished with the marine reflection survey apparatus previously described by periodically firing the seismic source when it and the trailing sensor are approximately equidistant from an intermediate reflection point. A set of traces for each such reflection point is collected, each successive trace being generated while the seismic source is moving away from the particular reflection point. The seismic wave gathering each successive trace is reflected at an increasingly greater angle from the same reflection point. The set of traces gathered for a particular reflection point by common depth point shooting may be combined or "stacked" by subsequent data processing. In stacking, a normal moveout correction function is applied to the gathered traces to remove the phase differences arising from the slightly differing path lengths of the incident and reflected seismic waves producing each trace. An additional correction is required to remove the effect of surface and near surface irregularities. This correction, called the "static correction", may vary in an irregular manner from point to point both for the receiving array and for each shot. Stacking a series of traces generated by common depth point shooting and corrected for normal moveout and statics produces a composite trace representing an "ideal" vertical reflection of a seismic wave in which the ratios of the primary reflection return signal strength to noise and to secondary return signal strength are improved over any of the traces being stacked.
A significant problem arises when the common depth point shooting method described is used to gather data in certain marine areas having irregular geophysical statics such as in the Gulf of Mexico near the mouth of the Mississippi River where the seafloor is covered by networks of closely spaced, randomly located channels filled to varying depths with mud. Compared with other materials commonly found under the seafloor, this mud has a very slow seismic wave propagation velocity. Thus, as incident and reflected waves or both pass through the mud, their propagation time is greatly delayed. These propagation delays appear as irregular phase shifts among the traces collected by standard common depth point shooting in such areas. The primary reflections in such irregularly phase shifted traces will not be enhanced by stacking. Moreover as the mud attenuates the magnitude of the seismic wave, the collected traces generally have signal to noise ratios so low as to prevent their processing prior to stacking to remove or otherwise correct those traces having the irregularly occurring phase shifts. The traces collected in such areas by conventional methods are essentially uninterpretable.
A method used in both land and marine survey to improve the signal to noise ratio of data is to deploy an array of suitable transducers and generate several seismic waves in succession at the same location so as to produce a plurality of traces from seismic waves propagating along the same paths. The sensors are then relocated and the process repeated. Gathers of traces may then be combined or stacked bu subsequent data processing. Placing and recovering a spread of individual transducers from the seafloor is an extremely laborious and time consuming task. The process can be sped up somewhat by the use of a cable incorporating a multiplicity of transducers. The cable is positioned at a first location to record a series of seismic shots and then moved to a new location for a new series of shots. While using a cable in this fashion is considerably faster than handling a plurality of individual sensors, it still requires considerably more support ship time than conventional marine reflection survey using a continuously moving source and sensor array. Moreover, if a ground cable is used it is subject to damages while being dragged over obstacles on the seafloor and other hazards such as trawlers.